This invention relates to the preparation of copper phthalocyanine pigment and, particularly, to an improved process for producing copper phthalcyanine in the .beta.-phase by viscous salt milling crude copper phthalocyanine.
It is well-known in the art that crude copper phthalocyanine, produced from conventional organic reactions, lacks adequate pigmentary properties such as high strength and texture for direct use as a pigment. Consequently, a variety of methods are commonly practiced to convert crude copper phthalocyanine to a useful pigmentary form. Two well-known distinct crystal phases of copper phthalocyanine, i.e., the .alpha.-phase and the .beta.-phase, can result from crude copper phthalocyanine depending upon the method used. The X-ray crystallographic definitions of the two phases of copper phthalocyanine are given in U.S. Pat. No. 2,556,730. In use, the .alpha.-phase exhibits a red-shade blue color and the .beta.-phase exhibits a green-shade blue color.
Commonly practiced conversion methods such as acid-pasting, i.e., dissolving crude copper phthalocyanine in concentrated acid then precipitating by dilution with water, and dry salt-milling, i.e., dry grinding crude copper phthalocyanine with a large excess of inorganic salt, invariably produce the .alpha.-phase. The .beta.-phase of copper phthalocyanine, which is the most stable phase, has been produced by a wide variety of methods, the most common of which are summarized below.
Ball milling crude copper phthalocyanine in the presence of organic liquids, preferably an oxygenated aliphatic liquid, to produce .beta.-phase copper phthalocyanine is described in U.S. Pat. No. 2,556,726. A related method known as salt-solvent milling is described in U.S. Pat. Nos. 2,556,728 and 2,556,730. Salt-solvent milling comprises milling a limited quantity of organic liquid with crude copper phthalocyanine and a large excess of salt. Another method described in U.S. Pat. No. 3,051,718 comprises acid-swelling a ball-milled chlorine-free copper phthalocyanine, utilizing a concentrated acid having a dissociation constant less than about 5 .times. 10.sup.-2 such as phosphoric.
Although the above-described methods can produce satisfactory pigmentary copper phthalocyanine in the .beta.-phase, these methods employ large quantities of inorganic salts, organic liquid or concentrated acid, are not very economical, and can require additional processing steps, e.g., separation of the pigment from the organic liquid by steam stripping.
One attempt to overcome the disadvantages of the prior art is described in U.S. Pat. No. 2,982,666. The method, known as viscous milling or viscous salt milling, comprises preparing a mixture of crude pigment, optionally inorganic salt, and organic conditioning agent to form a discontinuous mass of tough, compacted granular particles, then subjecting the mass of particles to grinding by internal shearing action which develops an energy input more than four times that required for grinding crude pigment in the absence of the conditioning agent and more than four times that required for grinding a mixture if it were in the form of a pasty mass, and extracting the ground mass with water.
Although the conditioned pigments prepared by the method of U.S. Pat. No. 2,982,666 may exhibit improved strength, the conditioned pigment may not necessarily be in the .beta.-phase and the energy required to provide sufficient shearing action to produce the conditioned pigment, whatever form, is quite high. For example, it is disclosed in U.S. Pat. No. 2,982,666 that relatively low shear mixing devices such as Werner-Pfleiderer mixers, which employ double mixing blades of the so-called "sigma" type, are incapable of exerting sufficient shearing action and that heavy duty dispersion mixers such as the Baker-Perkins dispersion mixer and Banbury mixer are required.
This invention provides for an improved viscous salt milling process which produces high quality .beta.-copper phthalocyanine and eliminates the need for especially high shear and consequently requires much less energy than conventional viscous salt milling.